Method for preparing single crystal pseudobinary alloys

ABSTRACT

A method for producing homogeneous single crystal ingots of pseudobinary alloys having high-crystal perfection by rapidly quenching a homogeneous liquid mixture of the desired constituents to produce a homogeneous polycrystalline ingot, then very slowly passing the polycrystalline ingot through a thermal gradient unitl the ingot is at a maximum temperature just below the solidus termperature of the alloy, thus causing the grain boundaries to migrate through the ingot as a result of directional solid-state recrystallization, then slowly coolng the ingot.

tates tet 72] Inventor Rowland E. Johnson Dallas, Tex. [2]] Appl. No. 788,171 [22] Filed Dec. 31, 1968 [45] Patented Nov. 23, 1971 [73] Assignee Texas Instruments Incorporated Dallas, Tex.

[54] METHOD FOR PREPARING SINGLE CRYSTAL PSEUDOBINARY ALLOYS 11 Claims, 5 Drawing Figs.

[52] 11.8. CI l48/l.6, 23/315, 75/135, 252/623 GA, 252/623 C, 252/623 ZT [51] Int. Cl B0lj 17/00, H011 7/00 [50] Field oiSearch l48/1.6; 75/135; 252/501, 62.3 GA, 62.3 ZT, 62.3 C; 23/315 [56] References Cited UNITED STATES PATENTS 3,351,502 11/1967 Rediker l48/1.6X 3,352,722 11/1967 Wanget al.... l48/l.6 3,410,665 1l/l968 Muller et al.

3,468,363 9/1969 Parker et al. 75/135 X 3,480,554 11/1969 Kendall et al 252/623 3,496,118 2/1970 Willardson et al. 148/ l .6 X

3,514,347 5/1970 Rodot et al. 148/186 OTHER REFERENCES Holden, A. N., Preparation of Metal Single Crystals,

Transactions of the ASM, Vol. 42, 1950, pages 319- 321 and 328- 333.

Primary Examiner-Hyland Bizot Assistant ExaminerG. K. White Altorneys--Samuel M. Mims, .Ir., James 0. Dixon, Andrew M.

Hassell, Harold Levine, Melvin Sharp, John M. Harrison and Richards, Harris and Hubbard ABSTRACT: A method for producing homogeneous single crystal ingots of pseudobinary alloys having high-crystal perfection by rapidly quenching a homogeneous liquid mixture of the desired constituents to produce a homogeneous polycrystalline ingot, then very slowly passing the polycrystalline ingot through a thermal gradient unitl the ingot is at a maximum temperature just below the solidus temperature of the alloy, thus causing the grain boundaries to migrate through the ingot as a result of directional solid-state recrystallization, then slowly coolng the ingot.

' I200 l I I soo- F TEMPERATURE C PATENTEDNUY 2 3 1971 M 'IZOO I I uJIIOO TEMPERATUR l l l l l TEMPERATUREC.

"x26 Row/and E. Johnson INVENTOR METHOD FOR PREPARING SINGLE CRYSTAL PSEUDOBINARY ALLOYS This invention relates generally to the preparation of pseudobinary alloys, and more particularly, but not by way of limitation, relates to the preparation of homogeneous, single crystal pseudobinary alloys which may be used in the manufacture of electronic devices.

Most semiconductor devices must be fabricated from very pure single crystal material because any imperfections in the semiconductor crystal adversely affect the electrical characteristics of the device. Several pseudobinary alloys of semiconductor materials offer considerable theoretical advantages for various semiconductor applications, but no satisfactory general process has heretofore been devised for producing single crystal pseudobinary alloys of sufficient size to make the fabrication of semiconductor devices commercially feasible.

For example, if a mixture of Hg,, :.,Cd Te, is liquified at a temperature slightly less than l,000 C. and then slowly cooled, fairly large single crystals can be produced. However, as the liquid mixture cools, the first-to-freeze part of the crystal is a substantially higher percentage cadmium telluride than the mixture, and the last-to-freeze part of the crystal is a substantially higher percentage mercury telluride than the mixture. The crystal will also have a continuous range of intermediate compositions. A solid ingot of substantially uniform composition can be produced by very rapidly quenching the liquid mixture, but this produces polycrystalline material having very small crystals so that the material is totally unsuited for most semiconductor applications. The crystal size in the polycrystalline ingot produced by quenching can be materially increased by a very long solid state recrystallization at a temperature just below the solidus temperature. However, crystal growth using this process is irregular so that the ingot is usually not single crystal, and also shows poor crystal perfection as determined by large Laue X-ray patterns, or by X-ray topographs. The imperfections may be dislocations, low-angle grain boundaries, and the like.

This invention is concerned with an improved method for producing large crystals of a pseudobinary alloy suitable for use in fabricating various semiconductor devices. In accordance with the invention, a mixture of the desired composition is melted and reacted, then very rapidly quenched to produce a homogeneous polycrystalline ingot of the desired composition. The ingot is then very slowly pulled through a temperature gradient to a final temperature just below the solidus temperature to produce directional recrystallization process which causes the grain boundaries to migrate from one end of the ingot to the other leaving a large ingot with a high degree of crystal perfection. In accordance with a more specific aspect of the invention, a very slight excess of one of the elements is provided to enhance the recrystallization process.

The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description of illustrative embodiments, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a freezing point diagram of a mercury telluridecadmium telluride pseudobinary alloy system;

FIG. 2 is a schematic diagram which illustrates the quenching step used to prepare a homogeneous polycrystalline ingot in the process of the present invention;

FIG. 3 is a sectional view of an ampul containing the ingot prepared in FIG. 2 preparatory to being subjected to directional recrystallization in accordance with the present invention;

FIG. 4 is a schematic drawing illustrating how the ampul of FIG. 3 is pulled through a furnace to produce directional recrystallization in accordance with the present invention; and

FIG. 5 is a temperature profile of the furnace of FIG. 4.

Referring now to the drawings, FIG. I is a freezing point diagram for a system of mercury telluride and cadmium telluride. The liquidus line 12 and solidus line 14 are approximations and are presented merely to illustrate the inherent problems associated with producing a single homogeneous crystal of a pseudobinary system. For purposes of illustration, assume that a liquid solution containing amounts of mercury, cadmium and tellurium representing 70 percent mercury telluride and 30 percent cadmium telluride is liquified at 1,000 C. As this liquid mixture is cooled, no solids are produced until a temperature of about 840 C. is reached, as represented by point 16 on the liquidus line 12. At this point, however, the first solids produced will have a composition corresponding to point 18 on the solidus line 14, which is about 70 percent cadmium telluride and 30 percent mercury telluride.

As the mixture is further cooled, the composition of the liquid stage will become increasingly rich in mercury and lean in cadmium as a result of the solidifying of greater proportions of cadmium telluride than mercury telluride. This progressivelylowers the freezing temperature along the liquidus line 12 with the result that the crystal is increasingly rich in mercury telluride. For example, when the liquid composition becomes percent mercury telluride and 20 percent cadmium telluride at point 20, so that the temperature is about 780 C., the crystals being formed will be about 58 percent mercury telluride and 42 percent cadmium telluride, as represented at point 22. Thus, although a fairly large crystal can be produced by slowly freezing a mixture of mercury-cadmium-tellurium, the crystal will be nonhomogeneous, varying in composition over a wide range. Such a crystal is not suitable for fabricating semiconductor devices. The crystal can be made more homogeneous by very rapidly quenching the liquid, but this results in a highly polycrystalline structure made up of very small crystals.

It is believed that the process of this invention is a general process applicable to substantially all pseudobinary systems involving semiconductor materials, and is particularly believed to be applicable to alloys formed from the compounds comprised of an element of Group III of the Periodic Table and an element of Group V, such as indium arsenide-indium antimonide (lnAs) .,(InSb),, to alloys formed from the compounds comprised of an element of Group IV and an element of Group VI, such as lead telluride-tin telluride (PbTe) ,(SnTe),, and to alloys formed from the compounds comprised of an element of Group II and an element of Group VI.

The process of this invention is particularly suited for producing an alloy of mercury telluride and cadmium telluride, and the specific process for producing a single crystal of this alloy is selected as an illustrative example. However, it is to be understood that the invention is believed to be applicable to the production of pseudobinary alloys generally. A mercury telluride-cadmium telluride ingot having a large number of very small crystals, each having the same average composition is prepared by sealing a nonstoichiometric composition of 10.200 grams of mercury, 1.519 grams of cadmium, and 8.279 grams of tellurium in a Spectrosil quartz ampul 24 while under a pressure of about 10 torr. This represents a composition having the mole ratio Hg Cd ,Te, The material was then reacted in a furnace at a temperature such as to insure that all components were in a liquid state, about 8 l4 C., for a period of about 18 hours. The ampul 24 was then very rapidly quenched using a jet of nitrogen gas 26 at room temperature at a flow rate of 50 liters per minute. The gas was directed up through the reactor to uniformly cool the ampul while the upper end of the ampul was kept from cooling by a cap of fiber quartz 27. This resulted in the highly polycrystalline ingot 28 that was relatively homogeneous.

The crystal 28 was subjected to a directional solid state recrystallization process in accordance with the present invention by placing the ingot in another slightly larger ampul 30 as illustrated in FIG. 3. The ingot 28 was sealed in the ampul 30 together with 0.325 grams of mercury 32, and separated from the mercury by a quartz spacer 34. The ingot 28 is inverted in the ingot 30 so that the last-to-freeze end 28a was at the lower end.

The ampul 30 was then suspended from a cable 35 and raised into a reactor indicated generally by the reference numeral 36 at a rate which will presently be described. The reactor was comprised of a vertical tube 38 surrounded by a plurality of resistive heating coils 40-43. The heating coils 40-43 were separately controllable and provided a temperature gradient as represented by curve 46 in the graph of FIG. 5, which is directly scaled to the vertical dimension of the schematic drawing of FIG. 4. The ampul 30 was slowly moved upwardly within the reactor 36 to the position shown in solid outline in FIG. 4 so that the upper end of the ingot 28 was at a temperature of about 6 C. and the lower end was at a temperature of about 525 C., giving a temperature differential of about 85 C. over the length of the ingot. After one day, the ampul 30 was again raised until the upper end of the ingot 28 was at approximately 650 C. and the lower end was at approximately 615" C., providing a temperature differential of 35 C. over the length of the ingot. Then the ampul 30 was raised at a very slow rate of about 0.33 mm./hour until the entire ingot 28 was at approximately the same temperature of 660 C. at position 301:. It is important that no part of the ingot be raised to a temperature above the solidus temperature. However, the nearer the ingot is brought to the solidus temperature, without exceeding the solidus temperature, the faster the rate of recrystallization. A period of about 10 days was required to move the ampul from position 300 to position 30b. The entire ingot was then cooled to room temperature at a rate of 30 C. per hour by cooling the furnace with the ingot in place. The resulting ingot was largely single crystal. X-ray topography and Laue photographs showed perfect crystal structure with no subgrains, dislocations, or other imperfections in slices taken from the last-to-freeze area.

In another instance, the same procedure was followed except that the composition used to produce the polycrystalline ingot 28 was 8.257 grams mercury, 1.143 grams cadmium and 6.481 grams tellurium plus 0.052 grams excess mercury to again produce the mole ratio Hg Cd Te plus the excess mercury. These materials were heated and reacted in a quartz ampul, then rapidly quenched. The ingot was then resealed in a second ampul together with a small quantity of mercury and a spacer as illustrated in FIG. 3, and recrystallized using the directional recrystallization process described above with substantially the same temperatures and substantially the same temperature gradients. The final material exhibited large single crystals free of blow holes and with good crystal perfection although there was evidence of some second phase material. While being significantly superior to material previously obtained by other processes, the material produced with excess mercury was inferior to that produced with excess tellurium as described in the previous example.

Alternatively, the ingot may be moved to a point which is at about 600 C. for a sufficient time to establish thermal equilibrium with the furnace, then moved continuously through the temperature gradient at the desired rate to the final isothermal temperature near the solidus temperature. The most important step of the invention is passing the thermal gradient through the solid-state ingot so as to produce a directional recrystallization to transport any secondary phase material and grain boundaries through the ingot and leave a single crystal of the desired composition.

Although preferred embodiments of the present invention have been described in detail, it is to be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. The method for producing large homogeneous single crystals of a pseudobinary alloy from a highly polycrystalline ingot which comprises slowly passing a temperature gradient from one end of the ingot to the other until all of the ingot is at approximately the same temperature just below the solidus temperature of the alloy.

2. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group 11 and an element from Group VI.

3. The method defined in claim 2 wherein the alloy is comprised of mercury telluride and cadmium telluride.

4. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group III and an element from Group V.

5. The method defined in claim 4 wherein the alloy is comprised of indium arsenide and indium antimonide.

6. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group IV and an element from Group V1.

7. The method defined in claim 6 wherein the alloy is comprised of lead telluride and tin telluride.

8. The method for producing a large, homogeneous, single crystal of a pseudobinary alloy from a highly polycrystalline ingot which comprises slowly moving the ingot relative to a thermal gradient until all of the ingot is heated to approximately the same temperature approaching the solidus temperature of the alloy.

9. The method for producing a large, homogeneous, single crystal of a pseudobinary alloy which comprises:

rapidly freezing a liquid mixture comprising about 1.0-(x) mole parts mercury, about (x) mole parts cadmium and greater than 1.0 mole parts tellurium to produce a polycrystalline ingot comprised of crystals of substantially the same average composition, then slowly moving the ingot relative to a thermal gradient until all of the ingot is heated to approximately the same temperature approaching the solidus temperature of the alloy.

10. The method of claim 9 wherein there is from about 1.00 to about 1.02 parts tellurium.

11. The method of claim 9 wherein there is about 0.79 mole parts mercury, about 0.21 mole parts cadmium, and between about 1.00 and about 1.02 mole parts tellurium. 

2. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group II and an element from Group VI.
 3. The method defined in claim 2 wherein the alloy is comprised of mercury telluride and cadmium telluride.
 4. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group III and an element from Group V.
 5. The method defined in claim 4 wherein the alloy is comprised of indium arsenide and indium antimonide.
 6. The method defined in claim 1 wherein the alloy comprises at least two compounds, each compound being formed by an element from Group IV and an element from Group VI.
 7. The method defined in claim 6 wherein the alloy is comprised of lead telluride and tin telluride.
 8. The method for producing a large, homogeneous, single crystal of a pseudobinary alloy from a highly polycrystalline ingot which comprises slowly moving the ingot relative to a thermal gradient until all of the ingot is heated to approximately the same temperature approaching the solidus temperature of the alloy.
 9. The method for producing a large, homogeneous, single crystal of a pseudobinary alloy which comprises: rapidly freezing a liquid mixture comprising about 1.0-(x) mole parts mercury, about (x) mole parts cadmium and greater than 1.0 mole parts tellurium to produce a polycrystalline ingot comprised of crystals of substantially the same average composition, then slowly moving the ingot relative to a thermal gradient until all of the ingot is heated to approximately the same temperature approaching the solidus temperature of the alloy.
 10. The method of claim 9 wherein there is from about 1.00 to about 1.02 parts tellurium.
 11. The method of claim 9 wherein there is about 0.79 mole parts mercury, about 0.21 mole parts cadmium, and between about 1.00 and about 1.02 mole parts tellurium. 